A significant prerequisite in nano-involved and bio-inspired approaches for the fabrication of electrical and optical devices or organization of building blocks to hierarchical structures, is the construction of well-ordered two-dimensional monolayers. It plays a key role not only in embedding a specific functionality on the focused area but also stacking to a multicomponent layer for further higher dimensional integration. Effects to date have primarily focused on two main methods: self-assembly monolayer (SAM) and Langmuir-Blodgett monolayer (LBM).
The self-assembly monolayer technique is based on the formation of covalent or coordination bonding between assemblable species and surface. It can generate dense and stable monolayers and serve as a template for bottom-up based nano-assembly. Due to the requirement of specific binding, however, the range of material applicability is limited to some chemical species, such as those amenable to thiol or silane chemistry. The Langmuir-Blodgett monolayer technique utilizes the hydrophobic-hydrophilic interactions at the interface between specific molecules and water surface. In this technique, molecules are spread to form a monolayer over the water surface and then transferred to the other processible solid surface. By varying an external force to guide the monolayer spreading, molecular density and orientation can be tuned as desired. In spite of these advantages of controllability in film properties, LBM has shown some drawbacks typically in film processiblity and stability due to the use of fluidic and unstable water surface.
Electrostatic interaction has been one of the important driving forces for molecular self-assembly. The most widely accepted technique is layer-by-layer (LBL) assembly. The technique takes advantage of electrostatic attractive forces between charged polymers and oppositely-charged surfaces, and film growth is typically achieved stepwise by the repetitive exposure of substrate to dilute polycation and polyanion solutions. Using this approach it is possible to control film thickness on the nanometer scale by simply increasing the number of adsorbed polycation/polyanion layers, or to fabricate films possessing gradients of different polyelectrolyte components by manipulating the sequences in which multiple different polymer components are adsorbed (G. Decher, Science, 1997, 277: 1232-1237; F. Caruso et al., Science, 1998, 282: 1111-1114; Z. Tang et al., Nature Mater., 2003, 2: 413-418; C. S. Peyratout and L. Dahne, Angew. Chem. Int. Ed. Engl., 2004, 43: 3762-3783). This assembly process has the added advantage of strong compatibility with biomolecular species without loss of biological function. Thus, LBL assembly has a wide variety of potential applications, including surface modification, sensors, conducting or light-emitting devices, drug delivery, nano-reactors, etc.
However, the formation mechanism, internal structure, and molecular properties of LBL-produced polyelectrolyte multilayers are still poorly understood, and this has hindered taking full advantage of this powerful technique.